Golf ball

ABSTRACT

An object of the present invention is to provide a golf ball having excellent scuff resistance and spin performance, and a satisfactory rebound property. The golf ball has a core  3,  an intermediate layer  5  covering the core  3,  an inner cover  7  covering the intermediate layer  5,  and an outer cover  15  covering the inner cover  7,  wherein the outer cover  15  includes a neutralized product containing an ethylene-unsaturated carboxylic acid-alkyl(meth)acrylate ternary copolymer having a weight average molecular weight (Mw) of 80,000 to 500,000 and an ethylene-acrylic acid or ethylene-methacrylic acid copolymer having a weight average molecular weight (Mw) of 2,000 to 30,000, and the inner cover  7  also includes the neutralized product.

TECHNICAL FIELD

The present invention relates to a golf ball.

BACKGROUND ART

Ionomer resin and polyurethane resin are generally used as main components of the material for the outer cover, i.e., the outermost layer, of a golf ball. Among these, golf balls using polyurethane resin (urethane golf balls) are often used by professional golfers and like upper-level players due to their excellent scuff resistance and spin performance (see, for example, Patent Literature 1).

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Publication No. 2004-97581

SUMMARY OF INVENTION Technical Problem

However, the urethane golf balls described above have drawbacks in that they exhibit a lower rebound property and a slower initial speed than golf balls that use ionomer resin in the outer cover (ionomer golf balls). In order to improve the rebound property, techniques such as thinly forming the outer cover have been developed. However, the rebound property is still insufficient compared to ionomer golf balls. Furthermore, urethane golf balls posed problems in the mass production and production cost thereof as they required a technique for thinly forming the outer cover. Therefore, the present invention aims to provide golf balls having excellent scuff resistance and spin performance, and, at the same time, a desirable rebound property.

Solution to Problem

The golf ball of the present invention comprises a core, an intermediate layer formed so as to cover the core, an inner cover formed so as to cover the intermediate layer, and an outer cover formed so as to cover the inner cover. The outer cover comprises a neutralized product containing an ethylene-unsaturated carboxylic acid-alkyl(meth)acrylate ternary copolymer having a weight average molecular weight (Mw) of 80,000 to 500,000, and an ethylene-acrylic acid or ethylene-methacrylic acid copolymer having a weight average molecular weight (Mw) of 2,000 to 30,000. The inner cover also comprises the neutralized product contained in the outer cover.

The use of ionomer resin in the outer cover makes the rebound property of this golf ball superior to that of golf balls that use polyurethane resin. Furthermore, because the inner cover also contains the ionomer resin that is used in the outer cover, the adhesion between the outer cover and the inner cover can be improved. As a result, scuff resistance can be enhanced and, at the same time, the rebound property can be improved by reducing the loss of energy from the striking force when hit by a driver or the like. Furthermore, the excellent rebound property of the ionomer resin enables the hardness to be reduced while maintaining the rebound property, and this can also improve the spin performance. Note that the neutralized product may be one obtained by preparing a mixture of a copolymer and an ethylene-unsaturated carboxylic acid-alkyl(meth)acrylate ternary copolymer and neutralizing the mixture using a base, one obtained by neutralizing an ethylene-unsaturated carboxylic acid-alkyl(meth)acrylate ternary copolymer and mixing a copolymer therewith, or one obtained by neutralizing a copolymer and mixing an ethylene-unsaturated carboxylic acid-alkyl(meth)acrylate ternary copolymer therewith. The weight average molecular weight (Mw) is measured by GPC to obtain the polystyrene equivalent molecular weight distribution.

In the above golf ball, the Shore D hardness of the inner cover is preferably about 62 to 72, and more preferably about 64 to 70. This enhances the ability of the inner cover to follow the deformation of the outer cover, further increasing the scuff resistance. The Shore D hardness of the outer cover is preferably about 50 to 57. The Shore D hardness of the inner cover and that of the outer cover are respectively measured for the surface of an inner cover that is formed on an intermediate layer, and for the surface of a ball in which an outer cover is formed on an inner cover. The Shore D hardness of the outer cover is measured at a portion where no dimple is formed. The Shore D hardness of a core and that of an intermediate layer are respectively measured for the surface of an as-formed core, and for the surface of an as-formed intermediate layer. More specifically, the Shore D hardness of a core is measured on the top end portion of a rib in the outward radial direction. However, if the flat surface portion is insufficient to measure the hardness, the measurement may be performed on a flat surface other than a rib on the core. This is because the same temperature and pressure are generally applied to the rib and portions other than the rib during the production of the core; therefore, the top end portion of the rib and other portions have substantially the same hardness. Regarding the intermediate layer, the measurement should be performed on a portion where a rib is not provided. The surface hardness measurement is conducted based on JIS K7215.

The thickness of the inner cover is preferably 0.9 to 1.2 mm, and the thickness of the outer cover is preferably 0.9 to 1.2 mm. By making the outer cover and inner cover thin, the core and intermediate layer can be enlarged. This suppresses spin when hit by a driver and improves the rebound property, thus achieving a longer carry distance. Note that the diameter of the spherical body composed of the above-described intermediate layer and core is preferably about 37.5 to 39.5 mm.

The core is provided with a spherical main part and a plurality of ribs formed on the surface of the main part. The intermediate layer may have a structure in which it is inserted into depressions surrounded by the ribs. By making the ribs harder than the intermediate layer, the ribs can limit the movable range of the intermediate layer in the struck portion that deforms in the direction along the spherical surface, thus preventing the striking force from being dispersed in the direction along the spherical surface. As a result, the striking force applied to the intermediate layer is efficiently transferred to the body of the core. This enables a high rebound property to be obtained, while also attaining a soft feeling when hit.

The scuff resistance can be further improved by making the edge angle of the plurality of dimples formed in the surface of the outer cover 6.2 to 7.2°.

Advantageous Effects of Invention

The present invention can provide a golf ball having excellent scuff resistance and spin performance and a desirable rebound property.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view showing a golf ball according to an embodiment of the present invention.

FIG. 2 is a perspective view of the core of the golf ball of FIG. 1.

FIG. 3 is a figure illustrating the edge angle of a dimple of the golf ball of the present embodiment.

FIG. 4 is a perspective view of another example of the core of the golf ball of FIG. 1.

FIG. 5 is a side view of the core of FIG. 4.

FIG. 6 is a side view of the principal parts of another example of the core of the golf ball of FIG. 1.

FIG. 7 is a side view of another example of the core of the golf ball of FIG. 1.

FIG. 8 is a side view of another example of the core of the golf ball of FIG. 1.

FIG. 9 is a side view of another example of the core of the golf ball of FIG. 1.

FIG. 10 is a figure showing an example of a manufacturing method using the core of FIG. 5.

FIG. 11 is a figure showing another example of a manufacturing method using the core of FIG. 5.

FIG. 12 is a cross-sectional view showing a golf ball according to another embodiment of the present invention.

FIG. 13 is a cross-sectional view showing the condition of the golf ball of the present invention when hit.

DESCRIPTION OF EMBODIMENTS

An embodiment of the golf ball of the present invention is explained below with reference to the drawings.

As shown in FIG. 1, the golf ball 1 of the present embodiment is a multi-piece golf ball comprising a core 3 covered with an intermediate layer 5, an inner cover 7 and an outer cover 15. According to the rules (see R&A and USGA), the diameter of a golf ball must be not less than 42.67 mm. However, taking aerodynamic characteristics and the like into consideration, it is preferable that the diameter of the ball be as small as possible. Therefore, it can be, for example, 42.7 to 42.9 mm. The core 3 is composed of a rubber composition, and, as shown in FIG. 2, comprises a spherical main part 9 and three ribs (protrusions) 11 molded as a unit on the surface of the spherical main part 9. Each rib 11 extends along one of the great circles drawn around the main part 9 so as to intersect each other at right angles. These ribs form eight depressions 13 in the surface of the main part 9.

The diameter of the main part 9 is preferably 34.0 to 36.0 mm, and more preferably 34.5 to 35.5 mm. The height of the ribs 11 is preferably 1.5 to 2.5 mm, and more preferably 1.75 to 2.25 mm. The Shore D hardness of the surface of the core 3 is preferably 50 to 60, and more preferably 53 to 58. By setting the hardness of core 3 within the above range, an excellent impact feel can be achieved while maintaining a sufficient rebound property.

As shown in FIGS. 1 and 2, each rib 11 is structured so as to have a trapezoidal cross section in such a manner that its width increases as it approaches the main part 9. The width of the top portion a of each rib in the outward radial direction is preferably 1.5 to 2.5 mm, and the width of the bottom portion b in the inward radial direction is preferably 3.0 to 6.0 mm. The widths of the end portions of the rib 11 may be set outside this range; however, by setting a lower limit for the width of each end portion of the rib 11, it is possible to prevent the rib 11 from being deformed due to the pressure of inserting the intermediate layer 5 that results from tightly closing the mold when the intermediate layer 5 is inserted during the manufacturing process. As a result, the core 9 can be accurately held in the center of the mold.

The intermediate layer 5 is made of a rubber composition or an elastomer, covers the surface of the core 3, and has a substantially spherical outside shape. As shown in FIG. 1, the intermediate layer 5 has almost the same thickness as the height of the ribs 11, and is inserted into each of the eight depressions 13 surrounded by the ribs 11. The top portions of the ribs 11 are exposed through the surface of the intermediate layer 5. In order to obtain a soft feeling when hit and improve the spin performance in an approach shot, it is necessary to make the hardness of the intermediate layer 5 lower than that of the core 3. The Shore D hardness of the intermediate layer 5 is preferably 47 to 57, and more preferably 50 to 55. In this structure, the Shore D hardness of the intermediate layer 5 is lower than that of the core 3 preferably by a value of 1 to 6.

The inner cover 7 is composed of an elastomer, and covers the top portions of the ribs 11 and the intermediate layer 5. The thickness of the inner cover 7 is preferably 0.7 to 1.5 mm, and more preferably 0.9 to 1.2 mm. The Shore D hardness of the surface of the inner cover 7 is preferably 62 to 72, and more preferably 64 to 70. The elastic modulus of the material for the inner cover 7 is preferably 300 to 500 Mpa, and more preferably 350 to 450 Mpa. The elastic modulus is measured in accordance with JIS K7016.

The outer cover 15 is composed of an elastomer, and covers the inner cover 7. Predetermined dimples (not shown) are formed in the outer surface of the outer cover. The thickness of the outer cover 15 is preferably 0.7 to 1.5 mm, and more preferably 0.9 to 1.2 mm. The Shore D hardness of the outer cover 15 is preferably 50 to 57, and more preferably 51 to 56. The thickness of the outer cover 15 is defined as the distance from an arbitrary point on the outermost part in the outward radial direction where no dimple is formed to an arbitrary point that comes into contact with the intermediate layer. Here, the measurement is performed along the normal line. The total thickness of the inner cover 7 and the outer cover 15 is preferably 1.5 to 3.0 mm, and more preferably 1.7 to 2.4 mm. Furthermore, the outer cover 15 preferably has a loss coefficient (tan δ) at −20° C. of 0.4 or less measured using a viscoelasticity analyzer (viscoelastic spectrometer) at a frequency of 10 Hz, a dynamic strain of 5%, tensile mode, and a rate of temperature increase of 4° C./min. The lower the loss coefficient (tan δ), the better the rebound property. The loss coefficient (tan δ) tends to increase as the dynamic strain increases; however, if the loss coefficient (tan δ) falls within the range of 0.1 to 0.4 even at a dynamic strain of 5%, a golf ball having an excellent rebound property and spin performance can be obtained.

The dimples formed in the outer cover 15 are explained below. The dimples can be circular or any of various polygonal, oval, or like shapes, and one type or a combination of two or more types can be used. For example, when circular dimples are used, the diameter may be 3.5 to 5.0 mm. The number of dimples is preferably 250 to 450. If too many dimples are provided, the trajectory of the ball lowers, and this may reduce the carry distance. On the other hand, if the number of dimples is too small, the trajectory of the ball rises, and this may also reduce the carry distance. The proportion of the area of the dimples relative to the total area of the spherical surface of the golf ball is preferably 70% or more, and more preferably 75% or more. The edge angle a of the dimple is preferably 6.0 to 7.5°, and more preferably 6.2 to 7.2°. By setting such a lower limit, an unduly large lift force can be prevented, and a desirable carry distance performance can be maintained. Setting the above upper limit can also maintain an excellent scuff resistance. As shown in FIG. 3, edge angle a of the dimple is the angle between line L1, which connects the ends of dimple D, and line T. Line T extends toward the dimple edge from intersection R of dimple D and line L2, which is offset 0.015 mm downward from line L1, and is tangent to the dimple arc.

The materials for each component of the above-described golf ball 1 are explained in detail below. The core 3 can be manufactured using a known rubber composition comprising a base rubber, a cross-linking agent, an unsaturated carboxylic acid metal salt, a filler, etc. Specific examples of the base rubber include natural rubber, polyisobutylene rubber, styrenebutadiene rubber, EPDM, etc. Among these, preferably used is high-cis polybutadiene that contains 80% or more cis-1,4 bonds.

Specific examples of cross-linking agents include dicumyl peroxide, t-butylperoxide and like organic peroxides; however, it is particularly preferable to use dicumyl peroxide. The compounding ratio of the cross-linking agent is generally 0.3 to 5 parts by mass, and preferably 0.5 to 2 parts by mass per 100 parts by mass of the base rubber.

As metal salts of unsaturated carboxylic acids, it is preferable to use monovalent or bivalent metal salts of acrylic acid, methacrylic acid and like C₃ to C₈ unsaturated carboxylic acids. Among these, the use of zinc acrylate can improve the rebound property of the ball and is particularly preferable. The compounding ratio of the unsaturated carboxylic acid metal salt is preferably 10 to 40 parts by mass per 100 parts by mass of the base rubber.

Examples of fillers include those generally added to cores. Specific examples thereof include zinc oxide, barium sulfate, calcium carbonate, etc. The preferable compounding ratio of the filler is 2 to 50 parts by mass per 100 parts by mass of the base rubber. If necessary, an antioxidant, a peptizer and the like may be added.

The intermediate layer 5 is composed of a rubber composition or elastomer as described above. When a rubber composition is used, the same materials as used for the core 3 described above can be used.

When the intermediate layer 5 is composed of an elastomer, it is possible to use, for example, a styrene/butadiene/styrene block copolymer (SBS), a styrene/isoprene/styrene block copolymer (SIS), a styrene/ethylene/butylene/styrene block copolymer (SEBS), a styrene/ethylene/propylene/styrene block copolymer (SEPS), or like styrene-based thermoplastic elastomer; an olefin-based thermoplastic elastomer having polyethylene or polypropylene as a hard segment and butadiene rubber, acrylonitrile butadiene rubber or ethylene/propylene rubber as a soft segment; a vinyl chloride-based plastic elastomer having crystallized poly(vinyl chloride) as a hard segment and amorphous poly(vinyl chloride) or an acrylonitrile butadiene rubber as a soft segment; a urethane-based plastic elastomer having polyurethane as a hard segment and polyether or polyester as a soft segment; a polyester-based plastic elastomer having polyester as a hard segment and polyether or polyester as a soft segment; a polyamide-based plastic elastomer having polyamide as a hard segment and polyether or polyester as a soft segment; an ionomer resin, etc.

It is preferable that the outer cover 15 use an ionomer resin as its main component. An example of such an ionomer resin is a neutralized product comprising an ethylene-unsaturated carboxylic acid-alkyl(meth)acrylate ternary copolymer having a weight average molecular weight (Mw) of 80,000 to 500,000, an ethylene-acrylic acid or ethylene-methacrylic acid copolymer having a weight average molecular weight (Mw) of 2,000 to 30,000, and a metal salt. One example of a metal salt is magnesium hydroxide.

The reason for using such a neutralized product is described below. A copolymer having a high weight average molecular weight is excellent in physical properties, such as the rebound property and scuff resistance, but such a copolymer has poor moldability. Therefore, a copolymer having a low weight average molecular weight, which has good flowability, is used in combination. These materials have similar structures and high compatibility with each other. Therefore, their combination enables a material having excellent moldability, rebound property and scuff resistance to be obtained. Furthermore, by adding a metal salt, the degree of neutralization of carboxylic acid can be increased and the rebound property can be enhanced accordingly.

Here, the weight average molecular weight (Mw) of the neutralized product is measured by GPC (gel permeation chromatography) and calculated as a polystyrene equivalent molecular weight. However, the weight average molecular weights of binary copolymer and ternary copolymer are immeasurable as they are; therefore, a sample thereof is dissolved in a mixed solvent of xylene and butanol by adding hydrochloric acid and heating the mixture. The weight average molecular weight of the obtained substance is then measured by letting the resulting solution reprecipitate in methanol.

The neutralized product may be one obtained by neutralizing, using a metal salt, a mixture of a copolymer and an ethylene-unsaturated carboxylic acid-alkyl(meth)acrylate ternary copolymer; one obtained by neutralizing an ethylene-unsaturated carboxylic acid-alkyl(meth)acrylate ternary copolymer using a metal salt and mixing a copolymer therewith; or one obtained by neutralizing a copolymer using a metal salt and mixing an ethylene-unsaturated carboxylic acid-alkyl(meth)acrylate ternary copolymer therewith. The main component of the outer cover 15 refers to a material that is contained in the largest amount of weight among the materials for the outer cover 15. For example, the proportion of the main component of the outer cover 15 is preferably about 60 to 100% by weight when the total weight of the outer cover 15 is defined as 100%. Specific examples of ionomer resins preferably used as the main component of the outer cover 15 include HPC AD1043 and HPC AD1022 both manufactured by Dupont. The outer cover 15 may contain other ionomer resins such as Himilan 1706 and Himilan 1605, manufactured by Dupont Mitsui Chemical Co., Ltd.; and Surlyn 9910, Surlyn 8940, Surlyn 8150, Surlyn 8120, and Surlyn 8320, manufactured by Dupont.

The inner cover 7 preferably contains the aforementioned ionomer resin that serves as the main component of the outer cover 15 in an amount of about 10 to 50% by weight, and more preferably about 15 to 45% by weight. The inner cover 7 may further contain other ionomer resins such as an ethylene-(meth)acrylic acid binary copolymer having an acid content of 10% or more. Specific examples thereof include Himilan 1706 and Himilan 1605, manufactured by Dupont Mitsui Chemical Co., Ltd.; and Surlyn 9910, Surlyn 8940, Surlyn 8150, HPF 1000, and HPF 2000, manufactured by Dupont. By forming the inner cover 7 so as to contain the main component of the outer cover 15 in a proportion within the range described above, the adhesion between the inner cover 7 and the outer cover 15 can be improved.

The golf ball 1 of the present embodiment having the above structure uses an ionomer resin instead of a urethane resin as the main component of the outer cover 15. This allows the golf ball 1 to exhibit an excellent rebound property. Having an excellent rebound property makes it possible to achieve remarkable spin performance by reducing the hardness while maintaining the rebound property. Because the material for the inner cover 7 contains the main component of the outer cover 15, adhesion between the outer cover 15 and the inner cover 7 can be enhanced; therefore, the deformation of the inner cover 7 follows the deformation of the outer cover 15 when hit. As a result, the scuff resistance can be improved and the loss of energy in the striking force when hit by a driver or the like can be reduced, further improving the rebound property. Because the material for the inner cover 7 includes the main component of the soft outer cover 15, the hardness of the inner cover 7 can be reduced and the spin performance in an approach shot can be improved accordingly. Here, by making the diameter of the spherical body composed of the core 3 and the intermediate layer 7 larger, reduction of the rebound property can be prevented.

When the hardness of the inner cover 7 is reduced to improve the spin performance in an approach shot as described above, the spin amount tends to undesirably increase in a conventional golf ball. This often results in the reduction of carry distance even when hit by a driver. However, in the present invention, by providing the ribs 11 on the core 3 and inserting the intermediate layer 5 into the depressions surrounded by the ribs 11, the spin amount can be reduced by using the restoring force of the ribs 11 that were deformed due to the shot by a driver.

More specifically, as shown in FIG. 13( a), in this golf ball, because the intermediate layer 5 is inserted into depressions surrounded by the ribs 11, when the ball is hit by a club C, the ribs 11 greatly deform. Due to the impact, a force causing backspin B is applied to the ball. When the ball separates from the club C, as shown in FIG. 13( b), the rib 11 returns to its original condition from the deformed condition, and this restoration applies a force F to the ball in the direction that cancels the backspin B. This reduces the amount of spin and increases the shot angle, further lengthening the carry distance. In particular, in the present embodiment, the ribs 11 are not simple protrusions but are structured so as to form walls surrounding the intermediate layer 5. Therefore, the restoring force of the ribs 11 acts strongly from the perimeter of the intermediate layer 5 via the entire wall surface, and this increases the force F opposite to the backspin B. This reduces the amount of backspin and achieves a significantly longer carry distance. This effect is particularly remarkable when the ball is hit by a driver, etc., which is designed to obtain a long carry distance. In FIG. 13, the current condition is shown by the solid lines and the condition immediately before the current condition is shown by the dashed lines. This makes it possible to obtain both excellent spin performance in an approach shot and a long carry distance when hit by a driver.

The above-described ribs may be formed into various shapes; however, from the viewpoint of effectively molding the intermediate layer, it is preferable to provide notches in the ribs as described below. FIG. 4 is a perspective view of a core provided with notches. FIG. 5 is a cross-sectional view of the core of FIG. 4. As shown in FIGS. 4 and 5, the notches 24 are structured so as to have a bottom surface 24 a extending along a tangent plane H that passes through the intersections P of the great circles. In other words, the notch 24 is formed by excising the rib 11 at the tangent plane H. By forming notches 24 in this manner, four depressions 13 that are arranged so as to have a common center at an intersection P of each of the great circles are made to communicate with each other, and the material for the intermediate layer can be readily spread between the depressions 13 via the notches 24. In this case, as shown in FIG. 6, the bottom surface 24 a of the notch 24 may be formed along a plane H₁ that extends away from the tangent plane H by being slanted toward the center of the rib 11 by 1 to 3°, i.e., a plane having an angle made between the normal line n of the main part 9 passing through the intersection P is 91 to 93° as viewed from the front. This arrangement enables the angle to serve as a draft angle, and, for example, when a core is molded using two molds, such as an upper mold and a lower mold, the core 3 can be easily removed from the mold.

When the notch 24 is formed as described above, the length of the notchless top portion of each arc section S of the ribs 11 in the arc direction, which is divided at the intersection P as shown in FIG. 5, is preferably 10 mm or more.

As shown in FIG. 7, it is also possible to form a notch 24 so as to have a bottom surface 24 a extending along a plane H₂ that is perpendicular to the normal line n that passes through the mid point in the height direction of the rib 11. In this case, in order to smoothly spread the intermediate layer material throughout the depressions 13, the distance D from the top portion of the virtual rib 11 without a notch 24 to the bottom surface 24 a is preferably 1.2 mm or greater. The length L should preferably be 10 mm or more, as in the above-described case. Furthermore, a draft angle can be formed by forming the bottom surface 24 a of the notch 24 along a plane that has an angle of 91 to 93° relative to the normal line n in the same manner as shown in FIG. 6.

A notch can also be provided in the middle of the arc section S of the rib 11 in the arc direction. More specifically, as shown in FIG. 8( a), the notch 25 may be formed so as to have two bottom surfaces 25 a each extending toward the intersection P from a point on the normal line m of the main body 9 that passes through the mid point Q of each arc section in the circumferential direction. In this case, the angle formed between the bottom surface 25 a and the normal line m is preferably 45 to 48° as viewed from the front. This arrangement makes it easy to remove the core 3 from the mold as described above. If this angle exceeds 48°, the above-described length L of the rib in the circumferential direction becomes unduly short. The depth D of the notch 25 is preferably 1.2 mm or more. The depth D may be set outside of this range; however, by setting the depth D in this range, the intermediate layer material can be smoothly spread throughout the depressions 13. Note that the depth D of the notch 25 is defined as the length from the top of the virtual rib 11 without a notch 25 to the deepest portion of the notch 25.

Alternatively, as shown in FIG. 8( b), the notch 25 can be structured so as to have two side surfaces 25 b each extending in the direction of the intersection P from a point Q on a normal line m of the main part 9 that passes through the mid point of each arc section S in the arc direction, and a bottom surface 25 c of an arc shape along the main part 9 connecting the two side surfaces 25 b. In this case, as in the case shown in FIG. 8( a), taking the draft angle into consideration, the angle between the side surface 25 b and the normal line m is preferably 45 to 48° as viewed from the front. Note that the bottom surface 25 c can also be formed so as to pass through the mid point in the height direction of the rib 11. Also in this case, the depth D of the notch is preferably 1.2 mm or more. As long as the easy removal of the core is ensured, two or more notches 25 may be provided in the middle of the arc section S.

As shown in FIG. 9, the arc section S may have a notch 24 as shown in FIG. 5, 6, or 7, and a notch 25 as shown in FIG. 8. As shown in FIGS. 8 and 9, the length L (=L₁+L₂) of the arc section S without a notch is preferably 10 mm or more.

In the above-described embodiment, the thickness of the intermediate layer 5 and the height of the rib 11 are the same; however, they do not necessarily have to be the same. For example, the thickness of the intermediate layer 5 may be greater than the height of the rib 11. However, it is preferable that the thickness of the intermediate layer 5 be slightly greater than the height of the rib 11, for example, by 0.3 mm or less.

One example of a method for manufacturing a golf ball having the above-described structure is explained next with reference to drawings. Here, a manufacturing method wherein an intermediate layer is formed from a rubber composition is explained. FIGS. 10 and 11 show the method for manufacturing a four-piece golf ball comprising the core of FIG. 5.

First, a core is molded. Here, a predetermined amount of non-vulcanized rubber composition is placed in a mold. As described above, this rubber composition comprises a base rubber, a cross-linking agent, a metal salt of unsaturated carboxylic acid, a filler and the like those mixed by a Banbury mixer, a roller or like mixing equipment. The rubber composition is then press molded at 130 to 180° C. to obtain a core 3 as shown in FIG. 4.

As shown in FIG. 10, an intermediate layer 5 is then formed by press molding. As shown in FIG. 10( a), the mold for the intermediate layer comprises an upper mold 43 and a lower mold 45, each having a hemispherical depression 41. The depressions 41 of the upper mold 43 and lower mold 45 have the same kind of roughly finished surfaces as those of the molds for the core. Around each depression 41, a plurality of depressions 49 for holding excess flow are formed.

As shown in FIG. 10( a), a non-vulcanized rubber composition 61 is inserted into the depression 41 of the lower mold 45, a rubber composition 61 is placed on the above-obtained core 3, and the core 3 is positioned between the upper mold 43 and the lower mold 45. Subsequently, as shown in FIG. 10( b), the upper mold 43 and the lower mold 45 are brought into contact. The rubber composition 61 is subjected to full vulcanization at 130 to 180° C. for 5 to 25 minutes and press molding, to obtain an intermediate layer 5.

At this time, the rubber compositions 61 placed on the core 3 and in the depression 41 of the lower mold 45 fill the depressions 13 while being pressed against the surface of the core 3. As described above, adjacent depressions 13 communicate with each other through notches 24, and therefore the rubber composition spreads throughout each depression and uniformly fills the space therein. The intermediate layer 5 may also be molded by injection molding using, for example, a mold such as that shown in FIG. 11. In this case, if no notches are provided, it is impossible to uniformly insert the rubber composition into each depression 13 without providing a gate for each depression 13. However, by providing notches 24 in the ribs 11, the rubber composition can be uniformly inserted into the depressions 13 via the notches 24 in the same manner as described above even when the rubber composition is inserted from a single gate 50 after placing the core 3 into the molds 47 and 48.

When the molding of the intermediate layer 5 is completed, the core 3 covered with the intermediate layer 5 is removed from the mold. Thereafter, an inner cover 7 is applied to the surface of the intermediate layer 5 by press molding or injection molding. Thereafter, an outer cover is applied to the surface of the inner cover by press molding or injection molding in such a manner that the cover has predetermined dimples, thus obtaining a golf ball of the present embodiment.

As described above, notches 24 are provided in the ribs 11, and adjacent depressions 13 communicate with each other through the notches 24. Therefore, the rubber composition 61 spreads throughout the depressions 13 and uniformly fills the space therein when pressed from any position on the surface of the core 3. The makes it possible to cover the core 3 with an intermediate layer in a single press molding step. As a result, the manufacturing time can be significantly reduced.

A method for manufacturing a golf ball comprising an intermediate layer with notches is explained above. A golf ball without notches can also be manufactured by almost the same method. However, when notches are not provided, it is necessary to conduct press molding after arranging the intermediate layer material so as to spread throughout the depressions, or, to conduct injection molding after providing a plurality of gates corresponding to the depressions.

One embodiment of the golf ball of the present invention is explained above. However, the golf ball of the present invention is not limited to this embodiment, and various modifications can be made as long as they do not depart from the scope of the invention. For example, three ribs are formed along great circles drawn around the main part in the present embodiment. However, the embodiment of the ribs is not limited to this and the shape, number and location thereof may be appropriately modified as long as depressions, to which an intermediate layer is inserted, can be formed by the ribs.

In the above embodiment, the core 3 is explained as being provided with ribs 11. However, the core 3 may be composed of the spherical main part 9 without forming any ribs as shown in FIG. 12.

EXAMPLE 1

The present invention is explained in further detail with reference to Examples and Comparative Examples. The scope of the present invention is not limited to the Examples described below. Here, 21 types of golf balls of the Examples of the present invention are compared to 5 types of golf balls of Comparative Examples. Excluding Example 21, each of the golf balls of the Examples and Comparative Examples has a shape as shown in FIG. 1. The golf ball of Example 21 has a shape as shown in FIG. 12.

Table 1 below shows the shape, material and the like for each golf ball. All the golf balls were manufactured to have a diameter of about 42.70 mm, a weight of about 45.50 g, and 366 dimples. In Table 1, “hardness difference” means the difference in hardness between the core and the intermediate layer, and “total thickness” means the total thickness of the inner cover and the outer cover. The diameter of the intermediate layer means the diameter of the intermediate layer including the core. “HPC” stands for HPC AD 1022 (an ionomer manufactured by DuPont), “8940” stands for Surlyn 8940 (manufactured by DuPont), “8150” stands for Surlyn 8150 (manufactured by DuPont); and “8320” stands for Surlyn 8320 (manufactured by DuPont). The unit for the proportions of the materials is % by weight.

TABLE 1 Core Intermediate layer Inner cover Outer cover Sueface Shore Main body Height Sueface Shore Hardness Sueface Shore Sueface Shore Total Edge angle D hardness diameter of rib D hardness diameter difference Material D hardness Thickness Material D hardness Thickness thickness of dimple Example 1 55 34.9 1.8 52 38.5 3 8940:HPC = 68 1.1 HPC:8150 = 54 1.0 2.1 6.8 7:3 8:2 Example 2 58 34.9 1.8 52 38.5 6 8940:HPC = 68 1.1 HPC:8150 = 54 1.0 2.1 6.8 7:3 8:2 Example 3 53 34.9 1.8 52 38.5 1 8940:HPC = 68 1.1 HPC:8150 = 54 1.0 2.1 6.8 7:3 8:2 Example 4 56 34.9 1.8 55 38.5 1 8940:HPC = 68 1.1 HPC:8150 = 54 1.0 2.1 6.8 7:3 8:2 Example 5 56 34.9 1.8 50 38.5 6 8940:HPC = 68 1.1 HPC:8150 = 54 1.0 2.1 6.8 7:3 8:2 Example 6 55 34.9 1.8 52 38.5 3 8940:HPC = 70 1.1 HPC:8150 = 54 1.0 2.1 6.8 8.5:1.5 8:2 Example 7 55 34.9 1.8 52 38.5 3 8940:HPC = 64 1.1 HPC:8150 = 54 1.0 2.1 6.8 5.5:4.5 8:2 Example 8 55 34.9 1.8 52 38.5 3 8940:HPC = 68 1.1 HPC:8150 = 56 1.0 2.1 6.8 7:3 7:3 Example 9 55 34.9 1.8 52 38.5 3 8940:HPC = 68 1.1 HPC:8150 = 51 1.0 2.1 6.8 7:3 9.5:0.5 Example 10 55 34.9 1.8 52 38.5 3 8940:HPC = 68 1.1 HPC:8150 = 54 1.0 2.1 6.2 7:3 8:2 Example 11 55 34.9 1.8 52 38.5 3 8940:HPC = 68 1.2 HPC:8150 = 54 1.2 2.5 6.8 7:3 8:2 Example 12 55 34.9 1.8 52 38.5 3 8940:HPC = 68 0.9 HPC:8150 = 54 0.8 1.7 6.8 7:3 8:2 Example 13 59 34.9 1.8 52 38.5 7 8940:HPC = 68 1.1 HPC:8150 = 54 1.0 2.1 6.8 7:3 8:2 Example 14 52 34.9 1.8 52 38.5 0 8940:HPC = 68 1.1 HPC:8150 = 54 1.0 2.1 6.8 7:3 8:2 Example 15 55 34.9 1.8 49 38.5 6 8940:HPC = 68 1.1 HPC:8150 = 54 1.0 2.1 6.8 7:3 8:2 Example 16 55 34.9 1.8 52 38.5 3 8940:HPC = 63 1.1 HPC:8150 = 54 1.0 2.1 6.8 5:5 8:2 Example 17 55 34.9 1.8 52 38.5 3 8940:HPC = 68 1.1 HPC:8150 = 57 1.0 2.1 6.8 7:3 6:4 Example 18 55 34.9 1.8 52 38.5 3 8940:HPC = 68 1.3 HPC:8150 = 54 1.3 2.6 6.8 7:3 8:2 Example 19 55 34.9 1.8 52 38.5 3 8940:HPC = 68 0.8 HPC:8150 = 54 0.8 1.6 6.8 7:3 8:2 Example 20 55 34.9 1.8 52 38.5 3 8940:HPC = 68 1.1 HPC:8150 = 54 1.0 2.1 5.8 7:3 8:2 Example 21 55 34.9 0 52 38.5 3 8940:HPC = 68 1.1 HPC:8150 = 54 1.0 2.1 6.8 7:3 8:2 Comparative 55 34.9 1.8 52 38.5 3 8940:8320 = 68 1.1 HPC:8150 = 54 1.0 2.1 6.8 Example 1 8:2 8:2 Comparative 55 34.9 1.8 52 38.5 3 8940:HPC = 68 1.1 8320:8150 = 54 1.0 2.1 6.8 Example 2 7:3 7:3 Comparative 55 34.9 1.8 52 38.5 3 8940:8320 = 71 1.1 HPC:8150 = 54 1.0 2.1 6.8 Example 3 9.5:1.5 8:2 Comparative 55 34.9 1.8 52 38.5 3 8940:8320 = 68 1.1 HPC 50 1.0 2.1 6.8 Example 4 8:2 Comparative 55 34.9 1.8 52 38.5 3 8940:8320 = 68 1.1 HPC:8150 = 54 1.0 2.1 7.6 Example 5 8:2 8:2

Table 2 below shows the composition (unit: parts by mass) of the materials for the core of each golf ball. Table 3 shows the composition (unit: parts by mass) of the materials for the intermediate layer of each golf ball. In Tables 2 and 3, BR-01 (product name, manufactured by Japan Synthetic Rubber Company) was used as cis-1,4-polybutadiene. Calcined Zinc Oxide (product name, manufactured by HakusuiTech Co., Ltd.) was used as the zinc oxide; precipitated barium sulfate (product name, manufactured by Sakai Chemical Industry Co., Ltd.) was used as the barium sulfate; Antage W-400, 2,2′-methylene bis-(4-methyl-6-tert-butyl phenol) (product name, manufactured by Kawaguchi Chemical Industry Co., Ltd.) was used as the antioxidant; Actor ZA (product name, manufactured by Kawaguchi Chemical Industry Co., Ltd.) was used as the zinc acrylate; and PERCUMYL D (product name, manufactured by NOF Corporation) was used as the dicumyl peroxide. Molding was performed under the conditions of a mold temperature of 160° C. and a crosslinking time of 6 minutes.

TABLE 2 Examples 1, 6-12, and 15-21 Comparative Examples Examples Example Example 1-5 Example 2 Example 3 4-5 13 14 Cis-1,4- 100.00 100.00 100.00 100.00 100.00 100.00 polybutadiene Zinc oxide 3.00 3.00 3.00 3.00 3.00 3.00 Barium sulfate 19.85 19.20 20.30 19.70 18.9 20.6 Antioxidant 0.10 0.10 0.10 0.10 0.10 0.10 Zinc acrylate 25.80 27.80 24.60 26.30 28.30 23.80 Dicumyl 1.50 1.50 1.50 1.50 1.50 1.50 peroxide

TABLE 3 Examples 1-3, 6-14, and 16-21, Comparative Examples 1-5 Example 4 Example 5 Example 15 Cis-1,4- 100.00 100.00 100.00 100.00 polybutadiene Zinc oxide 3.00 3.00 3.00 3.00 Barium sulfate 22.50 21.40 23.10 23.40 Antioxidant 0.10 0.10 0.10 0.10 Zinc acrylate 24.70 27.40 22.90 22.00 Dicumyl 1.50 1.50 1.50 1.50 peroxide

TABLE 4 1W Re- Initial SW bound speed Spin Carry Spin Scratch Impact co- (m/s) (rpm) (y) (rpm) score feel efficient Example 1 62.2 2500 198 5500 4 5 0.775 Example 2 62.3 2600 197 5600 4 4 0.776 Example 3 62.1 2400 197 5400 4 5 0.774 Example 4 62.3 2600 197 5500 3.5 4 0.776 Example 5 62.1 2700 196 5700 4 5 0.774 Example 6 62.4 2400 200 5200 3.5 4 0.777 Example 7 62.1 2700 196 5800 4.5 5 0.774 Example 8 62.2 2400 198 5500 4.2 4 0.775 Example 9 62.1 2700 196 5800 3.5 5 0.774 Example 10 62.2 2500 197 5500 4.5 5 0.775 Example 11 62.1 2600 196 5500 4 4.5 0.774 Example 12 62.4 2300 200 5200 4 5 0.777 Example 13 62.4 2800 192 5700 4 3 0.777 Example 14 62.0 2400 192 5300 4 4.5 0.773 Example 15 62.0 2800 192 5800 4 5 0.773 Example 16 62.0 2800 192 5800 4.5 5 0.773 Example 17 62.3 2400 199 5300 4.2 3 0.776 Example 18 62 2800 192 5600 3.5 4.5 0.773 Example 19 62.5 2200 200 4900 4 5 0.778 Example 20 62.2 2500 193 5400 4.5 5 0.775 Example 21 62.0 2500 193 5400 4 4.5 0.773 Comparative 62.0 2500 193 5400 2.8 5 0.773 Example 1 Comparative 62.0 2500 193 5300 2.5 5 0.773 Example 2 Comparative 62.4 2400 200 5000 2.5 3 0.777 Example 3 Comparative 62.0 2800 194 5800 2.8 5 0.773 Example 4 Comparative 62.0 2500 193 5400 2.8 5 0.773 Example 5

Table 4 above shows the results of a hitting test conducted using the golf balls of the Examples and Comparative Examples. In this test, the golf balls were hit using a hitting robot (manufactured by Miyamae Co., Ltd.: product name “SHOT ROBO V”) with a number 1-wood (1W: manufactured by Mizuno

Corporation, MP Craft 425, loft angle: 9.5°, shaft: QUAD 6 Butt Standard (shaft length: 45 inches, shaft flex: S)), and a sand wedge (SW: manufactured by Mizuno Corporation, MP T11, loft angle: 56°, nickel chrome plated, shaft: Dynamic Gold Wedge Flex, shaft length: 35.25 inches), and the initial speeds of the balls, carry distances, spin amounts, scuff scores, and impact feel were measured. Here, the head speed of the 1-wood was set at 43 m/s, and the head speed of the sand wedge was set at 17 m/s.

Regarding the scratch scores shown in Table 4, the surface condition of the ball after conducting the aforementioned sand wedge hitting test was visually checked by five people and evaluated using a 5-step rating (1: A dent remained in which the cover material was peeled off, 2: A large amount of fraying or scratching was obvious on the ball surface, 3: A small amount of fraying or scratching could be seen on the ball surface, 4: A slight amount of scratching could be seen on the ball surface (just enough to be visible to the naked eye), or 5: Almost no dent could be seen on the ball surface). The average of these values was used as the scratch score for each Example and Comparative Example. The higher the scratch score, the more resistant the ball was to scratches, i.e., the greater the scuff resistance. A test of the impact feel was conducted by five upper-level amateur golfers using a sand wedge. The five subjects were asked to select either 1: Very hard, 2: Hard, 3: Slight feeling of the core, 4: Soft, or 5: Very soft, to evaluate the feeling when the ball was hit. The average of these values was used as the impact feel value for each Example and Comparative Example and the results are shown in Table 4. The rebound coefficient was determined based on the ratio of the incident velocity to the rebound velocity when a ball fired from an air gun was struck against a steel plate at a speed of 43.7 m/s. As is evident from the comparison between the results of Examples 1 to 21 and those of Comparative Examples 1 to 5 shown in Table 4, all the golf balls whose inner cover comprised HPC, which was the main component of the outer cover, exhibited excellent scuff resistance.

EXPLANATION OF NUMERICAL SYMBOLS

-   1 golf ball -   3 core -   5 intermediate layer -   7 inner cover -   9 main part -   11 rib -   13 depression -   15 outer cover 

1. A golf ball comprising: a core; an intermediate layer formed so as to cover the core; an inner cover formed so as to cover the intermediate layer; and an outer cover formed so as to cover the inner cover, wherein the outer cover comprises a neutralized product containing an ethylene-unsaturated carboxylic acid-alkyl(meth)acrylate ternary copolymer having a weight average molecular weight (Mw) of 80,000 to 500,000 and an ethylene-acrylic acid or ethylene-methacrylic acid copolymer having a weight average molecular weight (Mw) of 2,000 to 30,000; and the inner cover comprises the neutralized product.
 2. The golf ball according to claim 1, wherein the inner cover has a Shore D hardness of 64 to
 70. 3. The golf ball according to claim 1, wherein the inner cover has a thickness of 0.9 to 1.2 mm, and the outer cover has a thickness of 0.9 to 1.2 mm.
 4. The golf ball according to claim 2, wherein the inner cover has a thickness of 0.9 to 1.2 mm, and the outer cover has a thickness of 0.9 to 1.2 mm.
 5. The golf ball according to claim 1, wherein the core comprises a spherical main part and a plurality of ribs formed on the surface of the spherical main part, and the intermediate layer is inserted into depressions surrounded by the ribs.
 6. The golf ball according to claim 5, wherein the ribs have a hardness greater than that of the intermediate layer.
 7. The golf ball according to claim 1, wherein a plurality of dimples are formed in the surface of the outer cover, and the dimples have an edge angle of 6.2 to 7.2°. 